The present invention relates to nonlinear optical frequency generators and, more particularly, to such a generator having an improved source for pumping radiation. Small, compact sources of optical radiation are desired for many purposes. For example, it is desired to have a compact source of "blue" light for use with certain biomedical instrumentation. It is also desirable to have "green" light for use with optical storage devices. Both blue and green light are referred to as forms of optical radiation--radiation having a frequency of a value falling within the typical frequency ranges referred to as the near infrared, the visible, and the ultraviolet ranges, i.e., radiation having a frequency of between about 0.2 .mu.m and 5 .mu.m.
One way of obtaining optical radiation of a desired frequency is to convert other radiation such as that of one or more frequencies issuing from a diode laser to the desired radiation. An example is the frequency "doubling" of a selected frequency by a nonlinear generator of a type referred to typically as a second harmonic generator (SHG). Reference is made to U.S. Pat. No. 5,036,220 naming applicant, among others, for an earlier invention in this field to which applicant made significant contribution.
One of the problems associated with this approach is that for efficient conversion it is desirable that the radiation issuing from the source diode laser be within highly prescribed frequency limits. One way to only obtain a selected frequency for the pump radiation (the diode radiation) is to provide a reflector which is made up of indices of refraction transitions selected to confine the source radiation output to the selected frequency. This is known. However, the approaches to obtaining it in the past have problems. Reference is made, for example, to Shinozaki et al. U.S. Pat. No. 5,247,528 which describes a nonlinear optical generator which utilizes quasi-phasematching to provide output radiation having the second harmonic wavelength of the incoming pump or, in other words, fundamental radiation. Shinozaki et al. teaches selecting periodic reversals of the spontaneous polarization for quasi-phasematching so that they will also provide desired transitions in the indices of refraction. Thus, Shinozaki et al. is constrained to the selection of transitions which will provide both functions. This constraint makes simultaneous optimization of frequency doubling and distributed transitions difficult.
Another approach is that described in Welch et al. U.S. Pat. No. 5,185,752. Welch provides periodic perturbations to select the wavelength of the frequency which is doubled. He-does so by etching the exposed portions of the top surface of the waveguide in the nonlinear material. This etching process is separate from the processes used to make the nonlinear waveguide and thus adds complexity to the fabrication of the device. Additionally, once the perturbations are etched into the waveguide material, they are permanent and a wrong selection cannot be corrected by reworking.
It should be noted that both of the above approaches involve a periodic variation in the effective index of a guided mode.